A short wavelength laser light source, such as a blue light source and a green light source, is being developed in the field of laser projectors, high-density optical storage devices, etc. The short wavelength laser light source is a laser light source that employs a system referred to as SHG (Second Harmonic Generation). The SHG system short wavelength laser light source converts infrared light of the fundamental wave oscillated by a semiconductor laser into second harmonics by a wavelength conversion element and outputs blue or green laser light.
Regarding the SHG system short wavelength laser light source, an optical device is being developed, in which a laser light emitting element and a wavelength conversion element are mounted on a silicon substrate. Such an optical device is well known as disclosed in, for example, Japanese Unexamined Patent Publication No. H6-338650 (paragraph 0013, FIG. 12, etc.). The optical device is also referred to as an optical module.
FIG. 4 is an external appearance perspective view of a conventional optical device (SHG device) 1, which is an SHG system short wavelength laser light source.
The optical device 1 has a plate-shaped single silicon substrate 10, and a laser light emitting element 20 and a wavelength conversion element 30 as optical parts. The laser light emitting element 20 and the wavelength conversion element 30 are optically coupled to each other and mounted on the silicon substrate 10 adjacent to each other. Optical coupling means that the positional relationship is determined for each other so that light emitted from one of the optical parts can enter the other optical part directly.
The laser light emitting element 20 is a chip-type semiconductor laser configured to emit infrared light, etc. The wavelength conversion element 30 is rectangular in shape and has an optical waveguide 31 inside thereof as indicated by the broken line. The optical waveguide 31 contains, for example, LN (lithium niobate: LiNbO3), which is a ferroelectric substance single crystal material, as its main component and to which MgO is added. The optical waveguide 31 is formed along the lengthwise direction at the center part of the wavelength conversion element 30.
When a drive current is supplied to the laser light emitting element 20 from outside, the optical device 1 emits infrared light L1, which is the fundamental wave. The infrared light L1 is input to the optical waveguide 31 of the wavelength conversion element 30 and converted into harmonics and green or blue laser light L2 is emitted. The laser light L2 emitted from the optical waveguide 31 of the wavelength conversion element 30 is transmitted to an external optical system by an optical fiber, etc., not illustrated schematically.
An outline of a method for mounting the laser light emitting element 20 and the wavelength conversion element 30, which are optical parts, on the single silicon substrate 10 when manufacturing such an optical device is explained by using FIG. 5 and FIG. 6.
FIG. 5 is an exploded perspective view of the optical device 1. FIG. 6 is a schematic longitudinal sectional view along VI-VI line in FIG. 4. However, for the silicon substrate 10, the laser light emitting element (LD) 20, and the wavelength conversion element (PPLN) 30, it is not necessary to illustrate their internal structures, and therefore, their sectional views are not illustrated.
As illustrated in FIG. 5, on the surface of the silicon substrate 10, bonding parts 40 and 50 for mounting the laser light emitting element 20 and the wavelength conversion element 30 are formed. As illustrated in FIG. 6, the bonding parts 40 and 50 have micro bump structures in which micro bumps (hereinafter, simply referred to as “bumps”) 41 and 51, which are a large number of small metallic projections configured by a metal material, such as gold (Au), are provided at predetermined pitches. In FIG. 6, in order to make the micro bump structure of the bonding parts 40 and 50 easy-to-understand, the bumps 41 and 51 are exaggerated in size.
On the undersurfaces of the laser light emitting element 20 and the wavelength conversion element 30, for example, Au films 22 and 32 are formed as metal films in the shape of a belt at the portions in opposition to the bonding parts 40 and 50, respectively, as illustrated in FIG. 6. However, the Au films 22 and 32 are not illustrated in FIG. 5.
The laser light emitting element 20 is arranged in a predetermined position on the bonding part 40 and pressure is applied thereto. Due to this, the laser light emitting element 20 is bonded to the silicon substrate 10 by surface activated bonding. Similarly, the wavelength conversion element 30 is arranged in a predetermined position on the bonding part 50 and pressure is applied thereto. Due to this, the wavelength conversion element 30 is bonded to the silicon substrate 10 by surface activated bonding. In this example, the bonding part 50 is formed as two parallel patterns not passing through the portion of the optical waveguide 31 illustrated in FIG. 4.
In more detail, the bumps 41 and 51 of the bonding parts 40 and 50 and the Au films 22 and 32 are cleaned by argon plasma before bonding and their surfaces are activated, respectively. Then, the laser light emitting element 20 and the wavelength conversion element 30 are adsorbed individually by a pressure tool, not illustrated schematically, and mounted on the bonding part 40 or the bonding part 50 of the silicon substrate 10. Then, a load (pressure load) is applied at normal temperature without heating. Consequently, the top surfaces of the bumps 41 of the bonding part 40 and the Au film 22 on the undersurface of the laser light emitting element 20, and the top surfaces of the bumps 51 of the bonding part 50 and the Au film 32 on the undersurface of the wavelength conversion element 30 come into contact with each other, respectively. Because of this, the bumps 41 and 51 are crushed slightly and surface activated bonding is performed. By the surface activated bonding, metal atoms and molecules of the bumps 41 and 51 of the bonding parts 40 and 50 and the metal atoms and molecules in the vicinity of the bonding surfaces of the Au films 22 and 32 are diffused toward the opposite sides and firm diffusion bonding is performed.
In the optical device such as this, if the positional relationship between the laser light emitting element 20 and the wavelength conversion element 30 optically coupled with each other are not adjusted with high precision, the optical coupling is not sufficient and it is no longer possible to input the infrared light L1 from the laser light emitting element 20 to the optical waveguide 31 of the wavelength conversion element 30 efficiently. Consequently, in order to obtain laser light with high output power, the center adjustment of the axis of emitted light of the laser light emitting element 20 and the axis of incident light of the wavelength conversion element 30 is very important.
Further, the wavelength conversion element 30 has the shape of a long plate, and therefore, if unwanted stress is applied when it is mounted onto the silicon substrate 10, the wavelength conversion element 30 is easily bent. If the wavelength conversion element 30 is bent, the optical waveguide 31 inside thereof is also bent, and as a result, transmission loss of the incident infrared light L1 increases and it is no longer possible for the wavelength conversion element 30 to function sufficiently.